This invention relates to methods for fabricating modules which interconnect integrated circuits with patterned layers of copper conductors that are separated by layers of polyimide; and more particularly, it relates to methods of coating the copper conductors such that they do not corrode.
In the prior art, one module of the type to which this invention relates is described in an article entitled "Copper/Polyimide Systems for High Performance Packaging", IEEE Transactions on Components, Hybrids & Manufacturing Technology, Volume 7, No. 4, December, 1984, by H. Vora, et al. As described in this article, the conductors are formed on an underlying layer of polyimide by sputtering thereon, (1) an unpatterned layer of chromium, (2) an unpatterned layer of copper on the chromium, and (3) an unpatterned layer of chromium on the copper. This Cr/Cu/Cr structure is then patterned in a conventional fashion by means of photoresist and a wet etch. Then the resulting patterned Cr/Cu/Cr conductors are covered with an overlying layer of polyimide. All of the process steps are then repeated to form additional Cr/Cu/Cr conductor layers which are separated by polyimide layers.
However, a problem with the above described interconnect structure is that no chromium covers the sidewalls of the patterned Cr/Cu/Cr conductors. Consequently, the copper in the sidewalls corrodes when it is covered by the overlying polyimide layer. This corrosion occurs because water is given off when the overlying polyimide layer is cured, and because the curing process occurs at a high temperature (e.g., 300.degree. C.).
To avoid this corrosion problem, silicon dioxide rather than polyimide could be used to cover the copper conductors. But silicon dioxide cannot be deposited over the conductors as smoothly as polyimide. Polyimide is spun on in a liquid form and then cured, and this spinning motion smooths out the polyimide surface. By comparison, silicon dioxide is deposited by a chemical vapor. An insulating layer with a smooth surface is very useful since it allows alternate layers of copper conductors and insulator to be stacked on top of each other many times.
It has been considered by the present inventor that it would be very desirable to be able to pattern a layer of copper conductors on the underlying polyimide layer, and then electrolessly plate just the exposed copper surfaces with a material that resists corrosion. However, prior art electroless plating methods plate the polyimide along with the copper. Consequently, the copper conductors become shorted together and thereby make the interconnect structure useless.
Accordingly, a primary object of the invention is to provide a method of fabricating a module which electrically interconnects integrated circuits with copper conductors on polyimide layers in which all of the above problems are resolved.